What skills are required to model / sculpt this head?
As already mentioned in the comments, make human is the shortcut.
A  shape created by this awesome tool in two minutes, limited in details but could be tuned wihtin blender:



Another way to re-create this with modelling either trace the contours of a background image. Or use subdividing, insets and proportinal editing:





After ripping V the mouth, the mesh could be easily adjusted to whatever head shape you want. Kudos to What&#39;s the best way to model organic shapes? 




You might also need this addon: http://wiki.blender.org/index.php/Extensions:2.6/Py/Scripts/Add_Mesh/Add_Gear